1. Field of the Invention
This invention relates to novel cationic phospholipids and methods for making them. This invention also relates to novel liposomes and aggregates comprising the phospholipids of the present invention that are useful for the delivery of nucleic acids and drugs to cells, both in vitro and in vivo. This invention also relates to the treatment of diseases by gene therapeutics involving transfection with DNA and introduction into cells of antisense nucleotides, as well as stable transfection with DNA engineered to become incorporated into the genome of living cells.
2. Description of the Related Art
The introduction of foreign nucleic acids and other molecules is a valuable method for manipulating cells and has great potential both in molecular biology and in clinical medicine. Many methods have been used for insertion of endogenous nucleic acids into eukaryotic cells. E.g., Graham and Van der Eb, Virology 52, 456 (1973) (co-precipitation of DNA with calcium phosphate); Kawai and Nishizawa, Mol. Cell. Biol. 4, 1172 (1984) (polycation and DMSO); Neumann et al., EMBO Journal 1, 841 (1982) (electroporation); Graessmann and Graessmann in Microinjection and Organelle Transplantation Techniques, pp. 3-13 (Cells et al., Eds., Academic Press 1986) (microinjection); Cudd and Nicolau in Liposome Technology, pp. 207-221 (G. Gregoriadis, Ed., CRC Press 1984) (liposomes); Cepko et al., Cell 37, 1053 (1984) (retroviruses); and Schaffner, Proc. Natl. Acad. Sci. USA 77, 2163 (1980) (protoplast fusion). Both transient and stable transfection of genes has been demonstrated.
Some of the first work on liposome delivery of endogenous materials to cells occurred some twenty years ago. Foreign nucleic acids were introduced into cells (Magee et al., Biochim. Biophys. Acta 451, 610-618 (1976), Straub et al., Infect. Immun. 10, 783-792 (1974)), as were foreign lipids (Martin and MacDonald, J. Cell Biol. 70, 515-526 (1976)), Proteins (Magee et al., J. Cell. Biol. 63, 492 (1974), Steger and Desnick, Biochim. Biophys. Acta 464, 530 (1977)), fluorescent dyes (Leventis and Silvius), and drugs (Juliano and Stamp, Biochem. Pharm. 27, 21-27 (1978), Mayhew et al., Cancer Res. 36, 4406 (1976), Kimelberg, Biochim. Biophys. Acta 448, 531 (1976)), all using positively charged lipids.
Of the many methods used to facilitate entry of DNA into eukaryotic cells, cationic liposomes are among the most efficacious and have found extensive use as DNA carriers in transfection experiments. See, generally, Thierry et al. in Gene Regulation: Biology of Antisense RNA and DNA, p. 147 (Erickson and Izant, Eds., Raven Press, New York, 1992); Hug and Sleight, Biochim. Biophys. Acta 1097, 1 (1991); and Nicolau and Cudd, Crit. Rev. Ther. Drug Carr. Sys. 6, 239 (1989) The process of transfection using liposomes is called lipofection. Senior et al., Biochim. Biophys. Acta 1070, 173 (1991), suggested that incorporation of cationic lipids in liposomes is advantageous because it increases the amount of negatively charged molecules that can be associated with the liposome. In their study of the interaction between positively charged liposomes and blood, they concluded that harmful side-effects associated with macroscopic liposome-plasma aggregation can be avoided in humans by limiting the dosage.
Felgner et al., Proc. Natl. Acad. Sci. USA 84, 7413 (1987), demonstrated that liposomes of dioleoylphosphatidylethanolamine (DOPE) and the synthetic cationic lipid N-[1-(2,3-dioleyloxy)propyl]-N,N,N-trimethylammonium chloride (DOTMA) are capable of both transiently and stably transfecting DNA. Rose et al., BioTechiques 10, 520 (1991), tested lipofection with liposomes consisting of DOPE and one of the cationic lipids cetyldimethylethylammonium bromide (CDAB), cetyltrimethylethylammonium bromide (CTAB), dimethyldioctadecylammonium bromide (DDAB), methylbenzethonium chloride (MBC) and stearylamine. All of the liposomes (except that with CTAB) successfully transfected DNA into HeLa cells. At high concentrations, however, CDAB and MBC caused cell lysis. Only DDAB was found to be effective in mediating efficient DNA transfection into a variety of other cell lines. Malone et al., Proc. Natl. Acad. Sci. USA 86, 6077 (1989), successfully transfected RNA, in vitro, into a wide variety of cells lines. Zhou and Haung, J. Controlled Release 19, 269 (1992), disclosed successful lipofection by DOPE liposomes stabilized in the lamellar phase by cationic quaternary ammonium detergents. The authors noted, however, that the relatively high cytotoxicity of these compounds would limit their use in vivo.
Hawley-Nelson et al., Focus 15, 73 (1990, BRL publications), disclosed the cationic lipid "LIPOFECTAMINE", a reagent containing 2,3-dioleyloxy-N-[2(sperminecarboxyamido)ethyl]-N,N-dimethyl-1-propanamini um trifluoroacetate (DOSPA). "LIPOFECTAMINE" was found to have higher transfection activity than several monocationic lipid compounds ("LIPOFECTIN", "LIPOFECTACE", and DOTAP) in six of eight cell types tested. They observed toxicity when both lipid and DNA were included in the same mixture.
Both Farhood et al., Biochim. Biophys. Acta 1111, 239 (1992), and Gao and Huang, Biochem. Biophys. Res. Comm. 179, 280 (1991), disclose cationic derivatives of cholesterol as components of liposomes capable of transfecting cells in vitro.
Liposomes comprising cationic lipids may also find use as carriers for gene therapy in in vivo applications. Some of the first in vivo applications of delivery of endogenous materials via liposomes was demonstrated twenty years ago. See, e.g., Straub et al., supra, and Magee et al., supra, (nucleic acids), and Mayhew et al., supra (drugs),
Holt et al., Neuron 4, 203 (1990), describe a DOTMA di-oleoxylphosphatidylethanolamine liposome that successfully transfected a vector expressing luciferase cDNA into embryonic brain of Xenopus in vivo.
Malone, Focus 11, 4 (1989, BRL publications), reported a similar study on Xenopus neural tissue as did Ono et al., Neurosci. Lett. 117, 259 (1990), in mouse brain.
Brigham et al., Am J. Med. Sci. 298, 278 (1989), disclosed intravenous injection of "LIPOFECTIN" and chloramphenicol acetyl transferase (CAT) plasmid into mouse lungs.
Nabel et al., Science 249, 1285 (1990), reported the expression of a .beta.-galactosidase gene in a specific arterial segment in vivo in Yucatan pigs by DNA transfection with cationic liposomes. Lim et al., Circulation 83, 2007 (1991), disclosed in vivo gene transfer of reporter genes (.beta.-galactosidase and luciferase) into arteries of dogs using cationic liposomes.
Hazinski, Sem. Perinatol. 16, 200 (1992) disclosed cationic liposome-mediated transfer of fusion reporter genes to the epithelial cells and transient protein expression via direct injection of DNA-liposome solution into the trachea.
Yosimura et al., Nucleic Acids Res. 20, 3233 (1992) demonstrated successful in vivo lipofection of the cystic fibrosis trans-membrane conductance regulator gene (CFTR) into airway epithelium of mice using the cationic liposome "LIPOFECTIN". Hyde et al., Nature 362, 250 (1993), also disclosed lipofection of CFTR using "LIPOFECTIN". They demonstrated successful delivery of the gene to epithelia of the airway and to alveoli deep in the lung of transgenic mice.
Several cationic amphiphiles have been reported as transfection agents. Ballas et al., Biochim. et Biophys. Acta 939, 8 (1988), reported the successful lipofection of tobacco mosaic virus RNA into tobacco and petunia protoplasts via liposomes composed of phosphatidylcholine (PC), cholesterol, and the hydroxyl form of the quaternary ammonium detergent diisobutylcresoxyethoxyethyldimethylbenzylammonium (DEBDA [OH.sup.- ]). Liposomes lacking the quaternary ammonium detergent practically failed to transfect the RNA. Importantly, Ballas et al. also observed that RNA and DNA complexed to liposomes bearing DEBDA[OH.sup.- ] were highly resistant to added RNAses and DNAses.
Pinnaduwage et al., Biochim. et Biophys. Acta 985, 33 (1989), disclosed the lipofection of pSV2 CAT plasmid DNA into mouse L929 fibroblasts using sonicated liposomes comprising DOPE and a quaternary ammonium detergent (dodecyl-, tetradecyl-, or cetyl-trimethylammonium bromide). Pinnaduwage et al. note, however, that a major drawback of using single chain amphiphiles such as detergents for drug delivery is their toxicity.
Taylor et al., Nucleic Acids Res. 20, 4559-4565 (1992) successfully transfected both RNA ribozymes and chimeric RNA-DNA ribozymes with "LIPOFECTIN".
Leventis and Sivius, Biochim. et Biophys. Acta 1023, 124 (1990), disclosed several cationic amphiphiles based on a hydrophobic cholesteryl or dioleoylglyceryl moiety whose hydrophobic and cationic portions are linked by ester bonds, which should facilitate degradation in animal cells. Leventis and Sivius demonstrated successful lipofection of plasmid pSV2 CAT into CV-1 and 3T3 cells using liposomes containing the cationic amphiphiles 1,2-dioleoyl-3-(4'-trimethylammonio)butanoyl-sn-glycerol (DOTB), DOTAP and cholesteryl (4'-trimethylammonio)butanoate (ChoTB).
Duzgunes and Felgner, Methods in Enzymology 221, 303 (1993), describe methods for transfection of nucleic acids. They teach that when preparing complexes of DNA and "LIPOFECTIN" for transfection, a net positive charge is desired, and the corresponding ratio of the weight of lipid to nucleic acid is about 4-10. They warn, however, that optimization should be undertaken for each cell line to be transformed.
The precise way in which nucleic acids and phospholipids (and other amphiphiles) interact and the structure formed before and during the transfection process is not well understood. Commonly, the nucleic acids are said to be entrapped within a lipid bilayer, which is the classic definition of "liposome." There is also a belief, however, that the nucleic acid does not become entrapped, but forms some other sort of aggregate with the phospholipids. See, e.g., Smith et al., Biochim. Biophys. Acta 1154, 327 (1993), for several models of lipid/nucleic acid interaction. Maccarrone et al., Biochem. Biophys. Res. Comm. 186, 1417 (1992), disclosed that liposome-DNA aggregate size and shape was a function of the ratio of the amount DNA to that of phospholipid. They concluded that DNA binds to the outer surface of liposomes, which then cluster into irregular spherical aggregates. They also noted that plasmid length had no effect on binding to liposomes. Gershon et al., Biochem. 32, 7143-7151 (1993) examined the fluorescence of ethidium bromide in the presence of DOTMA and DNA. They observed an abrupt drop in its fluorescence when DNA/ethidium bromide is titrated with DOTMA to the point of near electrical neutrality. Electron microscopy of the complex revealed an abrupt condensation of the DNA at the point of neutrality. It is evident from these experiments that, at least for DOTMA, but probably for most cationic lipids as well, that the structure of the complex changes at charge neutrality, and concomitantly the DNA becomes very compactly organized into a structure that is evidently quite different from a vesicular liposome. Legendre and Szoka, Pharm. Res. 9, 1235 (1992), studied in vitro lipofectlon using a DOTMA:DOPE liposome and concluded that the liposome probably uses at least two pathways to introduce DNA into cells: fusion with the plasma membrane and endocytosis.
It should be recognized that virtually all of the compounds described thus far in the literature as "cationic lipids" are, in fact, cationic amphiphiles or cationic detergents. The term "lipid" refers to a natural product. Most lipids contain fatty acids as their major hydrophobic component, although some (such as cholesterol, sphingolipids and polyisoprenoids) have other hydrophobic structures.
Reagent mixtures of phosphatidylethanolamine (PE) with either DOTMA or dioctadecyldimethylammonium bromide (DDMB) are commercially available (e.g., from Promega). DOTMA-based transfection reagents are expensive due to the synthetic complexity of the cationic lipid, while the simple detergents are cheap but require dilution of the cationic species with relatively expensive PE. Both show significant cytotoxicity (especially single-chain compounds). The underlying causes of the cytotoxicity are unclear, but the difficulty or impossibility of metabolizing these materials can only exacerbate this problem during long-term use. Cytotoxicity is not a pressing problem for transient transfection procedures, but it must be solved prior to the use of liposome transfection in therapeutic applications. Consequently, cheaper, safer, and more effective lipids useful in lipofection technology are desirable.
As is further described below, the present invention provides novel compounds having these attributes. This new class of compounds comprise phosphoglyceride derivatives having a modified phosphodiester linkage, wherein a non-bridging oxygen is alkylated, producing a phosphate triester. In so alkylating the phosphate moiety, the negative charge on the oxygen is eliminated.
Methylation of phosphodiester linkages to produce P(O)-methyl derivatives has been reported. Renkonen, Biochim. et Biophys. Acta 152, 114 (1968). Although treatment of phosphatidylcholine (PC) with diazomethane yields dimethyl phosphatidic acid with loss of the choline residue, O-methyl phosphatidylcholinium has been isolated in low yield from this reaction in the presence of triethylammonium hydrochloride as proton donor. Diazomethane on a preparative scale is extremely hazardous, however, and the number of readily available diazoalkanes is limited. We also note that methyl phosphatidylcholinium is relatively unstable. Thus, a more efficient method for producing phosphate triester derivatives of phosphatidylcholine is desirable.